Why does Mom's mobility decline so quickly when she comes home from rehab facility?

Rwarren, just say "no, i can't do that mom".

In Mom's case she had an armchair to sit in, when not in bed. Two armrests to push up with, and then a walker to lean on. At home she sits on the sofa, with one armrest to use. Mom refuses to go out and go to the place that sells medical devices; she will not have anyone come to the home to evaluate her needs. She expects me to go to the drugstore and bring her whatever random thing they sell there (some assembly required, just like Christmas) and isn't returnable. (And the physio from the home has already said that is not appropriate. Then again, she might be getting a commission from the medical-device store.)

artemis, she is playing you like a fiddle. Don't wait on her hand and foot.

I bet many an elder will do well in rehab because they know if they do then they can go home much quicker.

Then when at home, I wonder if the relatives are helicoptering over Mom, telling her to be careful, don't do this or that. Thus her self-confidence fades.

Dad hunches over his walker and we remind him, "Stand up straight!"
We have to kind of yell because he's 90 and can't hear very well.
Refuses to wear a hearing aid or stand up straight.
He did better at the rehab, too.

There are also more supports in a rehab facility, especially in the bathroom and along the walls. Even if homes are adapted, there still isn't the structural support, nor is there the medical support as well.

When my mom was in rehab I saw that the workers there are very no-nonsense, don't take no for an answer type of people.

Its like children who behave when they know an adult is watching and then as soon as they are alone they start to act up again. I'm not comparing your mom to a child but it could be that she feels like she can relax again when she is at home.

Maybe, she feels like she is expected to be more competent there and that they have high expectations and at home....well....she's able to be more honest about how she feels. Perhaps she's tired and it really takes a lot out of her. I'm just not a fan of pushing seniors who are very old, weak, poor balance and fall risk.

Also, let me add that sometimes I do think there is a psychological element to being around family, even if the person has dementia. My cousin is wheelchair bound, but is able to transfer from the chair to bed, car, etc. with the assistance of one person. However, I have noticed that when I stand nearby and she can see me, she has a very difficult time transferring. Getting her in and out of the transport van is almost impossible. So, I stepped away out of sight and she then was able to cooperate much more with the staff member who transports her. I'm not sure why, because her mental abilities are very impaired, but I do know that she does better when I'm not around. lol

Could it be that in rehab, she knows that she is surrounded by professional caregivers? And home, it's " just" her relatives?

If a doctor or anyone in a white coat tells my mom something, it's whole different story than if i do. You might want to consider assisted living, for just this reason.